There is a need for high refractive index materials in the electronics industry. Such materials are widely used in flat panel displays, CCD and CMOS image sensors, for example in light pipe structures, microlenses, and as color filter overcoats, where use of high-RI materials results in improved angular response and increased overall quantum yield. In general, these materials need to have an index of refraction (RI) above 1.6, and they need to be sufficiently transparent in the visible wavelength range (400 to 800 nm). They also need to have proper gap-filling and planarizing properties, as well as good thermal stability. To withstand common processing steps, they should be curable to yield solvent resistant films, which have good adhesion on surrounding materials. They should also be suitable for chemical mechanical polishing (CMP).
Conventionally, aromatic polymers containing phenyl groups and higher aromatic ring systems such as naphthalene, phenanthrene, and anthacene moieties have been used for these applications. They have relatively high refractive indexes, said indexes amounting to values between 1.50 and 1.65. As a rule of thumb, the larger the ring system, the higher is the RI of the material. However, ring systems having more than three fused aromatic rings have a problem: they will begin to absorb blue light. As the blue channel is commonly the least sensitive of the three photosites in the RGB (red-green-blue) matrix, the materials cannot be used as high refractive optical materials without further compromising the blue light performance of CMOS image sensor.
There exist some high refractive index sulfur-containing polymers, such as thioethers and thiocarbonates (reference 1). However, aliphatic sulfur-containing polymers have relatively poor thermal stability. On the other hand, high refractive index polyimides, like polyimides in general, need curing temperatures above 300° C., typically 350° C., which is sometimes too much for CMOS devices. Polyimides also tend to have yellow cast, limiting further their use in optical applications.
For the above reasons, there is continued need for high refractive index transparent materials that can be cured at or below 250° C. but which are nevertheless thermally stable up to 450° C.